Mass spectrometer

ABSTRACT

A mass spectrometer comprising an ionization chamber, an ion accelerator, a magnetic analyzer, and a collector for obtaining a mass spectrum, which collector is connected to an indicator; wherein a deflecting electrode for supplying accelerated ions to an electron multiplier at a certain definite cycle for a certain definite period is disposed between said ion accelerator and magnetic analyzer, which electron multiplier determines the total ion current.

United States Paten Nakajima Oct. 9, 1973 MASS SPECTROMETER [75]Inventor: Yasuo Nakajilna, Katuta-shi, Japan S f" g Lag/fence ssls anxammerurc Assigneei Hitachi, Tokyo, J p n Att0rneyCraig, Antonelli andHill [22] Filed: May 27, 1971 21 Appl. No.: 147,313 [57] ABSTRACT v Amass spectrometer comprising an ionization cham- [52] Us Cl 250/41 9 D250/41 9 ME 250/41 9 G ber, an ion accelerator, a magnetic analyzer, anda [51] 3'9/34 collector for obtaining a mass spectrum, which collec-[58] Fieid i 9 D tor is connected to an indicator; wherein a deflecting2 5 0 9 9 electrode for supplying accelerated ions to an electronmultiplier at a certain definite cycle for a certain definite period isdisposed between said ion accelerator [56] References cued and magneticanalyzer, which electron multiplier de- UNITED STATES PATENTS terminesthe total ion current. 2,947,868 8/1960 Herzog 250/419 ME 3,471,69210/1969 Llewellyn 250 419 0 Chums, 3 8 Figures MASS SPECTROMETERBACKGROUND OF THE INVENTION This invention relates to improvements inmass spectrometers, and more particularly to mass spectrometers in whicha device for deflecting the ion beam is disposed in the detector formeasuring the total ion current of a sample.

The mass spectrometer is a mass spectrum measuring instrument in whichthe sample to be analyzed is ionized and accelerated to form an ion beamfor analysis according to its mass by using an electric field ormagnetic field.

Recently, with rapid development in the industry in general, attainmentof higher accuracy in the measurement of mass spectrum has become amajor aim. Today, the spectrometer is required to be capable ofaccurately measuring even a very small peak of a mass spectrum. To thisend, the state of ionization, i.e., the state of how an ion is producedmust be accurately observed. In the conventional mass spectrometer, anion monitoring electrode is used and part of the ion beam is directed tothe electrode whereby the total ion current is measured.

In this type of spectrometer, the quantity of the ion beam directed tothe ion monitoring electrode must be sufficient to observe theionization state, i.e., to measure even a very small change in the totalquantity of ions. However, increasing the quantity of ions supplied tothe ion monitoring electrode decreases the magnitude of the ion beamdirected to the analyzer and, as a result, the spectrum analyzingaccuracy is lowered. The mass spectrometer of today is designed so thatan electric field or magnetic field produced in the mass analyzer ischanged, andthe ion absorbed by the collector through a certain specificslit is amplified whereby a mass spectrum is obtained. In such aspectrometer, the ratio of the quantity of the ion beam absorbed by theion monitoring electrode to the quantity of ion beam supplied to themass analyzer is kept constant. In spite of this arrangement, the ratioof the ion beam is varied when the electric field or magnetic fieldprovided in the mass analyzer is scanned. The ratio of the ion beam isvaried also byvthe magnetic field of the source magnet provided in theion source. In the prior art, said ratio of the ion beam is assumed tobe constant and the total ion is calculated from the ion current flowingin the ion monitoring electrode. Hence, the variation in the ratio ofthe ion beam exists directly as the measuring error and lowers themeasuring accuracy.

In some cases, an accurate measure of the total quantity of the ion beamis necessary depending upon the sample to be analyzed. As described, theion current is caught by the monitoring electrode and amplified by anamplifier. The amplifier often involves drift and internal noise and ishardly capable of maintaining a satisfactory amplification factor. Thismakes it difficult to obtain an accurate measure of the total quantityof the ion beam.

While, to increase the sensitivity, the use of an electron multiplier isdesirable. In the prior art, however, part of the ion beam is detected.In such a structure, the size of the electron multiplier is too large tofit in the apparatus. Furthermore, the main beam is deflected by a highvoltage (Usually, -l,000 to 3,000) applied between the electrodes of theelectron multiplier. This high voltage often affects normal operationfor detecting the mass spectrum.

Recently, the mass spectrometer has been in use in direct combinationwith the gas chromatograph. In this system, the total ion monitor of.the mass spectrometer is used for chromatographic measurement andrecording. According to the prior art, however, part of the carrier gasenters the ion chamber together with the sample, which causes a problemin that the carrier gas in the ion chambervaries with lapse of time,thereby to vary the base line of the chromatogram.

A method for solving this problem is proposed in US. Pat. No. 3,430,040.According to this method, part of the ion beam is caught by the ionmonitoring electrode and amplified by an electrical amplifier. The ratioof the ions caught by the ion monitoring electrode to the total ions ischanged by the magnetic field of the source magnet of the ion source, aswell as by the magnetic field of the mass analyzing magnet. The carriergas from the gas chromatograph is largely deflected by the magneticfield because the mass of the carrier gas is small. Furthermore, in theproposed method, an electrical amplifier is used to amplify the ioncurrent caught by the ion monitor and, as a consequence, a sufficientamplification factor can hardly be obtained. In addition, the prior artdoes not make simultaneous measurement of gas chromatograph and massspectrum available SUMMARY OF THE INVENTION The first object of thisinvention is to provide a mass spectrometer in which the ratio of thequantity of ions caught by the ion monitoring electrode to the totalquantity of ions is kept constant by the magnetic field of the sourcemagnet provided inthe ion source and of the mass analyzing magnet.

The second object of the invejtion is to provide a mass spectrometer inwhich an electron multiplier can be used for the monitoring amplifier.

The third object of the invention is to provide a mass spectrometer inwhich the ratio of the quantity of ions caught by the monitoringelectrode to the total quantity of ions can easily be changed with ahigh accuracy.

The fourth object of the invention is to provide a mass spectrometercapable of gas chromatographic measurement without being affected by thecarrier gas when the spectrometer is used directly with a gaschromatograph.

'IZhe fifth object of the invention is to provide a mass spectrometercapable of simultaneous measurement of a gas chromatograph and a massspectrum when the spectrometer is used in direct combination with a gaschromatograph.

The gas spectrometer realized with the above objects in view ischaracterized in that an electrode for deflecting the ion beam isdisposed in part of the ion beam path, a square-wave pulse voltage isapplied to said deflecting electrode, thereby intermittently deflectingthe ion beam from its path, and the deflected ion beam is caught by acollector disposed on the ion beam deflecting path. In this massspectrometer, the quantity of ions caught by the mass spectrum measuringcollector and that caught by the ion monitoring collector are determinedby the ratio of the periods of time for which the ions are deflectedtoward the individual collectors. In this structure, therefore, the ionbeam is not affected by the source magnet of the ion source or by themass analyzing magnet and thus the mass spectrum is accurately measured.

For accurate measurement of variation in the total ion current accordingto this invention, an electron multiplier is used for the monitoringcollector, and the collected ion current is amplified at a highamplification factor (e.g., As a result, even a very small variation inthe total ion current can be accurately measured.

In the mass spectrometer used in direct combination with a gaschromatograph, the electron accelerating voltage for ionization ischanged, being synchronized with the periodic deflection of ion beam,whereby a gas chromatograph and a mass spectrum are simultaneouslymeasured free from influence of the carrier gas.

More specifically, in the gas chromatograph, a carrier gas is used.Hence, if the sample is ionized at an accelerating voltage at which thecarrier gas is not ionized, only the sample to be anaylzed can beionized without ionizing the carrier gas. At such accelerating voltage,the total ions proportional to the quantity of the sample supplied fromthe column is measured by the ion monitor. According to this method,however, the accuracy of the mass spectrum measurement is loweredbecause the ionization rate is small. In the measurement of the massspectrum, the ions of the carrier gas are cut by the collector slit and,accordingly, the measuring accuracy is free of the influence of thecarrier gas. For the measurement of the mass spectrum, the ionizingvoltage must be high enough. While, for the measurement of the totalions by the use of an ion monitor, the ionizing voltage must be lowerthan the voltage at which the carrier gas is ionized.

Taking the above conditions into consideration, the invention realizesan apparatus comprising a mass spectrometer in direct combination with agas chromatograph, characterized in that the electron acceleratingvoltage is of a square-wave pulse voltage having two levels, one beingbelow the carrier gas emergence voltage'and the other being above thecarrier gas emergence voltage, the ion beam is deflected toward the ionmonitor side in synchronism with said square-wave pulse voltage whensaid electron accelerating voltage is below the carrier gas emergencevoltage, or the ion beam is not deflected but driven along its normalpath when said electron accelerating voltage is above the carrier gasemergence voltage and thus the base line drift of the gas chromatogramrecorded for the spectra detected by the ion monitor is compensatedwithout affecting the mass spectrometer in its sample analysisoperation.

The electron accelerating voltage is a voltage applied between thefilament and ionization chamber for the purpose of accelerating theelectrons when a certain substance is ionized by bombardment of thermalelectrons. Strictly, this voltage is somewhat different from theionizing voltage for providing electron energy for ionization. Forexplanatory simplicity, the electron accelerating voltage is consideredto be similar to the ionizing voltage in this specification.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram showingthe principle of a mass spectrometer embodying this invention,

FIG. 2 illustrates a mass spectrometer of this invention used in directcombination with a gas chromatograph, and

FIG. 3 illustrates an embodiment of this invention wherein a pulsevoltage is applied to an electron accelerator disposed in the ionizingmeans.

' DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, there isshown a mass spectrometer of this invention wherein there is provided anionziation chamber 1 into which a gasified sample is introduced via asample inlet 2. The reference 3 denotes an acceleration slit whosepotential is the same as that of the ionization chamber, and 4 a focusslit for focusing the ion beam on an exit slit 5 which stands at groundpotential. Generally, this focus slit 4 receives a voltage a littlelower than the voltage applied to the acceleration slit 3. An ion beamaccelerating voltage E0 (selectable in several steps in the rangebetween 450 and 3,600V according to the value of the measuring mass) isapplied between the accelerating slit 3 and exit slit 5. The reference 6represents a filament which is fixed-biased for a suitable electronaccelerating voltage E of -50 to l00V with respect to the potential ofthe ionization chamber 1. By this voltage E, (normally V), the electronsgenerated from the filament are accelerated and supplied to a trap 7 inwhich the given sample is ionized. This trap 7 is kept at a voltage(normally 30 to 40V) which is higher by voltage E than that of theionization chamber 1.

The sample introduced into the ionization chamber 1 via the sample inlet2 is ionized by said electron accelerating voltage, and the resultantion beam is analyzed for mass by a magnetic field generator 8 anddetected by an ion collector 10 through a collector slit system 9. Thedetected ion beam is amplified by a suitable amplifier system, and theresultant mass spectrum is recorded on a recorder.

The reference 14 denotes a deflecting electrode disposed immediatelyafter the exit slit 5. A positive voltage pulse E is applied to saiddeflecting electrode 14 from a square-wave pulse generator 13. Thereference 15 denotes an ion monitor/detector system comprising an iondetector, such as an electron multiplier. This system is disposed on thepath of the ion beam deflected by said deflecting electrode.

In the mass spectrometer arranged as above, the ion beam is deflectedtoward the ground side of the deflecting electrode 14 when thesquare-wave voltage applied to the deflecting electrode 14 from thesquare-wave pulse generator 13 is of positive voltage E The deflectedion beam enters the ion monitor/detector system 15.

While, when the square-wave voltage applied to the deflecting electrode14 is of zero potential (i.e., ground potential), the ion beam is notdeflected but directed to the magnetic field 8 whereby the ion beam isanalyzed with respect to mass. The ratio of the quantity of ionssupplied to the ion monitor/detector system to the quantity of ionssupplied to the analyzer system can be arbitrarily changed by changingthe pulse widths t, and t, of the square-wave voltage.

The square pulse voltage E may be suitably determined according to thelength of the deflecting plate of the deflecting electrode and also thedeflecting angle.

As described above, the mass spectrometer according to this invention isoperated in such manner that a square-wave pulse voltage having twolevels is applied to the deflecting electrode disposed at a positionalong the ion beam path, one of these two levels being sufficient todeflect the ion beam to a desired value and the other one of which beingat zero potential (ground potential), whereby the ion beam isintermittently deflected, and thus the ion is detected and recorded bythe use of an ion monitor/detector system. This spectrometer is capableof ion monitoring at a high sensitivity, without lowering the detectionsensitivity due to decrease in the quantity of ion supplied to theanalyzer system, unlike the prior art in which part of the ion beam iscut for monitoring.

According to the invention, an electron multiplier, whose amplificationfactor and sensitivity are great, can be used for the ionmonitor/detector system. In addition, the electron multiplier can beinstalled at a position distant from the main beam path. This makes itpossible to eliminate the problem that the main ion beam is deflected bythe high voltage applied between the electrodes of said multiplier.

In the above embodiment, the ion beam is formed of positive ions. It isapparent that the invention is not limited to positive ions but is alsoapplicable to a negative ion beam.

The above-described mass spectrometer is of the single convergence typein which the deflecting electrodeis located immediately after the exitslit of the ion source. It is apparent that the invention is alsoapplicable to the double convergence type in which the velocityconvergence of the ion beam in an electrostatic field and thedirectional convergence in a magnetic field are utilized. In thisapplication, the deflecting electrode may be located in a suitableposition between the electrostatlc field and the magnetic field.

Referring to FIG. 2, there is shown a mass spectrometer of thisinvention used in direct combination with a gas chromatograph. In thisembodiment, the electron accelerating voltage for ionization is changedin synchronism with ion beam deflection whereby the influence of thecarrier gas is removed.

In FIG. 2, the reference 1 denotes an ionization chamber into which thesample separated by the gas chromatograph is led from a sample inlet 2by way of a carrier gas separator. The reference 3 represents anacceleration slit kept at the same potential as the ionization chamber1, and 4 designates a focus slit for focusing the ion beam on an exitslit 5 which stands at ground potential. Generally, this focus slit 4receives a voltage which is a little lower than the voltage applied tothe acceleration slit 3. An ion beam accelerating voltage E0) selectablein several steps from 450 to 3,600V according to the value of themeasuring mass) is applied between the acceleration slit 3 and exit slit5. The reference 6 represents a filament which is fixedbiased for asuitable electron accelerating voltage E of -50'to l00V with respect tothe potential of the ionization chamber 1. By this voltage E (normally70V), the electrons are accelerated and supplied to a trap 7 in whichthe given sample is ionized. This trap 7 is kept at a voltage (normally30 to 40V) which is higher by voltage E, than that of the ionizationchamber 1.

The sample introduced into the ionization chamber 1 via the sample inlet2 is ionized by said electron accelerating voltage, and the resultantion beam is analyzed for mass by a magnetic field 8 and detected by anion collector 10 through a collector slit 9. The detected ion beam isamplified by a suitable amplifier system and then recorded as a massspectrum on a recorder.

The electron accelerating voltage E applied between the filament 6 andionization chamber 1 is sufficient for ionizing the sample gas andcarrier gas.

A square-wave voltage E supplied from a squarewave pulse generator 13 issuperposed on the DC voltage E with reverse polarity. By this means, theelectron accelerating square-wave voltage applied between the ionizationchamber and filament is provided in two levels, E (=7OV) and E E (=2OV).When the voltage (E -E is determined to be lower (for example, 20V) thanthe carrier gas emergence voltage (about 24V in case of helium), and thevoltage E is the normal electron accelerating voltage (for example, V),the carrier gas is not ionized during operation where (E -E is appliedas the electron accelerating voltage, or both the sample gas and carriergas are ionized during operation where E, is applied as the electronaccelerating voltage. Synchronizing with said electron acceleratingvoltage, the square-wave voltage from the square-wave pulse generator 13is applied to the deflecting electrode 15 disposed immediately after theexit slit 5. Namely, when the electron accelerating voltage is (E -E thepositive pulse of amplitude voltage E is applied to the deflectingelectrode 15. While, when the electron accelerating voltage E, isapplied thereto, the deflecting electrode 15 stands at ground potential.

According to this embodiment, the ion beam is deflected toward theground potential side of the deflecting electrode by the pulse voltageapplied to the deflecting electrode 15 when the electron acceleratingvoltage is below the carrier gas emergence voltage. The deflected ionbeam is detected by a total ion monitor 16, such as an electronmultiplier disposed on the path of the deflected ion beam. Then thedetected result is recorded as a gas chromatogram on the recorder.

When the electron accelerating voltage is above the carrier gasemergence voltage, the deflecting electrode 15 is kept at groundpotential and, therefore, the ion beam is not deflected but is directedto the magnetic field 8, whereby the ion beam is dispersed with respectto mass and detected by the ion collector 10 via the collector slit 9,and thus the mass spectrum is recorded.

It is now apparent that the ion beam incident on the total ion monitor16 does nto contain ions of the carrier gas, and, hence, no drift iscaused in the base line of the gas chromatogram obtained through thetotal ion monitor even if the flow of the carrier gas is varied due totemperature rise.

According to the invention, the sample gas is ionized at an electronaccelerating voltage higher than 50V. By using these ions, a massspectrum is obtained via the ion collector. As a result, the spectrum isfree of influences due to decrease in the quantity of ions when theelectron accelerating voltage is set at a value below the carrier gasemergence voltage.

' In the above embodiment, the electron accelerating voltage is selectedat two levels, 20V and 70V. Instead, a pulse of 50V may be added to a DCvoltage of 20V. FIG. 3 shows a circuit in which a 50V pulse is added toa 20V DC voltage. The terminal A stands at zero potential. A positivepulse of 50V is applied between the terminals A and B, and a positivepulse of V between the terminals A and C. The purpose of the capacitor Cis to block direct current and thus to keep the reference potential ofthe ionization chamber high, so as to sufficiently accelerate the ion.

The capacitors C and C are to block DC and to bias the input pulses at acertain specific voltage. When the voltage applied across the terminalsA and B is zero, the 20V across the resistor R is applied to theionization chamber 1 via the diode S, and, at the same time, thecapacitor C is charged. When the voltage applied between the terminals Aand B is changed to 50V, the voltage of 20V across the capacitor C isadded forward, and the resultant voltage of 70V is applied to theionization chamber 1. In other words, the voltage ap plied between thefilament 6 and ionization chamber is changed in the range between 20Vand 70V according to the pulse between the terminals B and C. Therefore,when a pulse of 50V is applied between the terminals B and C insynchronism with the pulse applied to the beam deflecting plate 15, theelectron accelerating voltage is changed in synchronism with thedeflecting operation of the deflecting plate 15.

Briefly, in the foregoing embodiment wherein the mass spectrometer isused in direct combination with a gas chromatograph, the electronaccelerating voltage is a square-wave pulse voltage having two levels,one being below the carrier gas emergence voltage and the other beingabove the carrier gas emergence voltage. A voltage sufficient to deflectthe ion beam is applied to the deflecting electrode disposed at aposition along the t ion beam path in synchronism with said square-wavepulse voltage when said electron accelerating voltage is below thecarrier gas emergence voltage, or said deflecting electrode is kept asground potential when said accelerating voltage is above the carrier gasemergence voltage whereby drift is eliminated from the base line of thechromatogram recorded according to the ions detected by the ion monitor.

The invention is not limited to the above example with respect to themethod of producing the squarewave pulse voltage used as the electronaccelerating voltage, or the method of generating the deflecting voltagesynchronized with said square-wave pulse voltage applied to thedeflecting electrode. It is apparent that any modification may be maderegarding these methods without departing the true spirit of thisinvention.

The distribution ratio of the ions directed to the total ion monitor andto the analyzing system may arbitrarily be determined by changing thepulse widths t, and t, (FIG. 1) of the square-wave voltage E In theabove embodiment, the ion beam is formed of positive ions. Instead, theion beam may be made up of negative ions for the purpose of thisinvention.

In the foregoing embodiments, the single convergence type massspectrometer is described, and the deflecting electrode is locatedimmediately after the exit slit of the ion source. It is apparent thatthe invention is applicable to the double convergence type massspectrometer utilizing the velocity convergence of ions in anelectrostatic field and the directional convergence of ions in a in amagnetic field. In this case, the deflecting electrode may be located ata suitable position between the electrostatic field and magnetic field.

As described above, the invention employs the method in which the ionbeam is intermittently supplied to the ion monitor for monitoring anddetecting the ions. According to this method, an electron multiplierwhose sensitivity and amplification factor are very large can be usedfor the ion monitor, and the ion monitor can be located away of the mainbeam path. Thus, undesirable influences clue to deflection of the mainbeam by the high voltage applied between the electrodes of said electronmultiplier are eliminated, unlike the conventional method in which acollector plate is disposed on the ion beam path, and part of the ionbeam is cut and amplified by a current amplifier.

What is claimed is:

1. In an apparatus for mass spectrum determination comprising anionization chamber including means for generating an ion beam from a gasand means for accelerating said ion beam along a beam path,

a magnetic analyzer disposed along said beam path,

indicator means for obtaining a mass spectrum of said gas from the ionsderived from said magnetic analyzer, the improvement comprising adetector disposed adjacent said ion beam path, deflecting means disposedbetween said ionization chamber and said magnetic analyzer to deflections from the ion beam toward said detector for ion monitoring, and

means for supplying a pulse voltage to said deflecting means to directportions of said ion beam alternately to said ion detector and saidmagnetic analyzer.

2. An apparatus in accordance with claim 1, wherein said detector is anelectron multiplier.

3. An apparatus in accordance with claim 1, wherein the pulse width ofthe pulse voltage applied to the deflecting means is variable wherebythe quantity of the ion introduced into the monitor means may becontrolled.

4. In an apparatus for mass spectrum determination comprising: a gaschromatograph, ionization means for ionizing a gas sample to form an ionbeam projected along a beam path, means for introducing a gas sampleinto said ionization means from said gas chromatograph, a magneticanalyzer disposed along said beam path, and an indicator for obtaining amass spectrum of said gas sample,

the improvement comprising monitor means for detecting ions, deflectingmeans disposed at a suitable position between said magnetic analyzer andsaid ionization means for deflecting said ion beam from said beam pathtoward said monitor means for measuring a gas chromatograph, and meansfor supplying a pulse voltage to said deflecting means to directportions of said ion beam alternately to said magnetic analyzer and saidmonitor means.

5. An apparatus in accordance with claim 4, wherein said ionizationmeans includes an electron beam generator and an electron accelerator,the accelerating voltage of the electron accelerator disposed in theionization means being variable.

6. An apparatus in accordance with claim 4, wherein the acceleratingvoltage of the electron accelerator disposed in the ionization means ischanged in two levels, one being higher and the other being lower thanthe voltage for ionizing the carrier gas introduced thereinto from saidchromatograph.

7. An apparatus in accordance with claim 6,,wherein the electronaccelerating voltage is changed in synchronism with the pulse voltageapplied to the deflecting means, when the ion beam is deflected by thedeflect ing means, the electron accelerating voltage is set at aionization means for ionizing a gas sample to form an ion beam projectedalong a beam path,

means for introducing a gas sample into said ionization means from saidgas chromatograph,

a magnetic analyzer disposed along said beam path,

an indicator for obtaining a mass spectrum of said gas sample, andmonitor means for ions, the improvement comprising deflecting meansdisposed at a suitable position between said magnetic analyzer and saidionization means for deflecting portions of said ion beam alternately tosaid magnetic analyzer and said monitor means and means for supplying analternating voltage to said deflecting means.

10. An apparatus in accordance with claim 9, wherein said monitor meanshas an electron multiplier at a position distant from the main beam pathwhere the main ion beam cannot be affected by a high voltage appliedbetween electrodes of said multiplier.

1. In an apparatus for mass spectrum determination comprising anionization chamber including means for generating an ion beam from a gasand means for accelerating said ion beam along a beam path, a magneticanalyzer disposed along said beam path, indicator means for obtaining amass spectrum of said gas from the ions derived from said magneticanalyzer, the improvement comprising a detector disposed adjacent saidion beam path, deflecting means disposed between said ionization chamberand said magnetic analyzer to deflect ions from the ion beam toward saiddetector for ion monitoring, and means for supplying a pulse voltage tosaid deflecting means to direct portions of said ion beam alternately tosaid ion detector and said magnetic analyzer.
 2. An apparatus inaccordance with claim 1, wherein said detector is an electronmultiplier.
 3. An apparatus in accordance with claim 1, wherein thepulse width of the pulse voltage applied to the deflecting means isvariable whereby the quantity of the ion introduced into the monitormeans may be controlled.
 4. In an apparatus for mass spectrumdetermination comprising: a gas chromatograph, ionization means forionizing a gas sample to form an ion beam projected along a beam path,means for introducing a gas sample into said ionization means from saidgas chromatograph, a magnetic analyzer disposed along said beam path,and an indicator for obtaining a mass spectrum of said gas sample, theimprovement comprising monitor means for detecting ions, deflectingmeans disposed at a suitable position between said magnetic analyzer andsaid ionization means for deflecting said ion beam from said beam pathtoward said monitor means for measuring a gas chromatograph, and meansfor supplying a pulse voltage to said deflecting means to directportions of said ion beam alternately to said magnetic analyzer and saidmonitor means.
 5. An apparatus in accordance with claim 4, wherein saidionization means includes an electron beam generator and an electronaccelerator, the accelerating voltage of the electron acceleratordisposed in thE ionization means being variable.
 6. An apparatus inaccordance with claim 4, wherein the accelerating voltage of theelectron accelerator disposed in the ionization means is changed in twolevels, one being higher and the other being lower than the voltage forionizing the carrier gas introduced thereinto from said chromatograph.7. An apparatus in accordance with claim 6, wherein the electronaccelerating voltage is changed in synchronism with the pulse voltageapplied to the deflecting means, when the ion beam is deflected by thedeflecting means, the electron accelerating voltage is set at a valuelower than the voltage at which the carrier gas is ionized, and when theion beam is free of the deflecting means, the electron acceleratingvoltage is set at a value higher than the voltage at which the carriergas is ionized.
 8. An apparatus in accordance with claim 7, wherein saidmonitor means includes an electron multiplier.
 9. In an apparatus formass spectrum determination comprising: a gas chromatograph, ionizationmeans for ionizing a gas sample to form an ion beam projected along abeam path, means for introducing a gas sample into said ionization meansfrom said gas chromatograph, a magnetic analyzer disposed along saidbeam path, an indicator for obtaining a mass spectrum of said gassample, and monitor means for ions, the improvement comprisingdeflecting means disposed at a suitable position between said magneticanalyzer and said ionization means for deflecting portions of said ionbeam alternately to said magnetic analyzer and said monitor means andmeans for supplying an alternating voltage to said deflecting means. 10.An apparatus in accordance with claim 9, wherein said monitor means hasan electron multiplier at a position distant from the main beam pathwhere the main ion beam cannot be affected by a high voltage appliedbetween electrodes of said multiplier.